Conventional construction of fuel cell and electrolyzer stacks, especially proton exchange membrane (PEM) stacks, require a large number of flat components (including bipolar plates, membrane and electrode assemblies, and, optionally, cooling plates) to be assembled between a pair of heavy metal endplates. The entire assembly is placed in compression through the use of a series of long threaded metal rods (tie rods) extending from one endplate of the assembly to the other endplate with nuts or other fasteners on either end.
A conventional electrochemical cell stack has a plurality of cells disposed between two endplates. The cell stack and endplates are compressed by extending metal rods from one endplate to the other endplate and fastening the ends of the rods, such as with bolts. This type of design is often referred to as a “filter press” design. While conventional “filter press” designs may be straight-forward, and effective, they are also bulky and heavy. In conventional “filter press” designs the entire load is applied by the bolts along the edges of the stack. In order to compress the stack as evenly as possible over the cross-sectional area of the stack without bending the end plates, the endplates are made very thick. While increasing the thickness of the end plates may help make them rigid, increasing endplate thickness results in an increased total weight of the electrolyzer stack. Another contributing factor to the increased weight in the “filter press” design is the placement of the tie rods around the perimeter of the active portion of the stack, thereby requiring endplates that are even larger in area than the stack.
The size of the metal end plates can be marginally reduced by placing the metal tie rods inside of gas passages. While this type of design allows reduction in electrolyzer stack-weight, the reduction is limited by the continuing need for heavy metal tie rods and endplates in this type of design. Furthermore, placing the metal tie rods within the gas passages or manifold promotes corrosion of the tie rods and allows electrolysis to occur along the tie rods because of the difference in potential between the tie rods and the electronically conducting components of the individual cells.
Another approach to stack compression is disclosed by Gibb et al. in U.S. Pat. No. 5,484,666. Here, the electrochemical cells have manifolds that extend through the membrane and electrode assemblies or active area of the cells and the metal tie rods are disposed within these manifolds. The primary benefit of this configuration is that the compression is more evenly distributed over the central portion of the cell so that the endplate thickness can be reduced. However, this marginal reduction in weight comes at the expense of complicating the design and manufacture of the active area. Furthermore, the tie rods take up a significant amount of the cross-sectional area of the manifold, requiring a corresponding increase in the manifold area. In order to accommodate this configuration while achieving a specified level of cell performance requires a corresponding increase in the overall dimensions of the active area.
Yet another approach is disclosed by Barton et al. in U.S. Pat. No. 6,190,793. This patent discloses an electrochemical fuel cell stack with an improved compression assembly comprising a tension member that is preferably rigid and electrically non-conductive, such as a composite material having a bonding agent and reinforcing fibers. In one embodiment, the compression assembly employs a collet and wedges to grip the tension member and compress a resilient member that imparts a tensile force to the tension member and compressive force to the fuel cell assemblies. While the composite materials disclosed by Barton et al. are lighter in weight than metal and avoid contamination of the cell through the introduction of metals, the tension member does not serve to reduce the overall cell dimensions.
Therefore, there is a need for a system that compresses stacks that allows minimal overall dimensions of the stack. Furthermore, it would be desirable to have a compression system that does not taking up so much of the volume of the stack. It would be desirable if the compression assembly could be placed within internal passages of the cell, such as the manifolds, without blocking the flow channels to the individual cells.